1. Field of the Invention
This invention relates to a serial printer for performing a bi-directional printing.
2. Description of the Related Art
In a general serial printer, the print head has a plurality of dot printing elements arranged in the paper feed direction at a preset pitch, and reciprocates in a direction perpendicular to the paper feed direction to scan paper. The printer produces bit map pattern data having dot rows whose number corresponds to the number of the dot printing elements, prints a dot image of characters or figures by driving the dot printing elements according to the bit map pattern data in the forward path of the print head, and feeds the paper by a distance corresponding to a printing height of the print head. In recent years, most serial printers are constructed to also print in the return path of the print head.
In a case where the height of characters or figures exceeds the printing height of the print head, the printer prints a dot image representing the upper half of characters, for example in the forward path of the print head, and a dot image representing the lower half thereof in the return path of the print head. However, the printing quality of the characters is deteriorated when a dot alignment error is caused by a difference in the printing direction.
A printer which can reduce the deterioration in the printing quality is disclosed in U.S. Pat. No. 5,044,796. With the above printer, it is checked whether the bit map pattern data contains a break line which separates characters or figures adjacent in the paper feed direction. If the break line is found within a range of dot lines corresponding to the number of dot printing elements of the print head, the height of the characters or figures is determined to be not larger than the printing height of the printing head. At this time, the print head scans paper one time to print the characters or figures in the form of a dot image whose height is limited based on the position of the break line. On the other hand, if the break line is not found within a range of dot lines corresponding to the number of dot printing elements of the print head, the height of the characters or figures is determined to be larger than the printing height of the printing head. At this time, the print head scans paper plural times to print the characters or figures in the form of a dot image whose height is equal to the printing height of the print head. The printing direction is reversed only when the break line is found. That is, the printer fixedly sets the printing direction in one direction so as to prevent the printing quality from being deteriorated.
A printer which can reduce the deterioration in the printing quality caused by a dot alignment error due to the precision of the paper feeding mechanism is disclosed in U.S. Pat. No. 4,999,646. With the above printer, bit map pattern data of one page is converted into first and second complementary pattern data items of the same matrix size as the bit map pattern data. The printer sets the distance of paper feed to be equal to half the printing height of the print head, and prints dot images in the forward path and return path of the print head. In the forward path of the print head, dot printing elements thereof are driven based on a portion of the first pattern data which corresponds to the scanning area of the print head. In the return path of the print head, the dot printing elements are driven based on a portion of the second pattern data which corresponds to the scanning area of the print head. That is, with the above printer, the scanning areas of the forward path and return path are overlapped by half the printing height of the printing head and dot images printed in the forward path and return path are combined to as to prevent the printing quality from being deteriorated.
However, in the printer disclosed in U.S. Pat. No. 5,044,796, the throughput thereof cannot be improved at all when the printing direction is fixed in one direction. Further, it is necessary to check whether or not the break line is contained in the bit map pattern data in order to determine the printing direction and the height of the dot image. The checking process requires a relatively long time and limits the enhancement of the throughput.
In the printer disclosed in U.S. Pat, No. 4,999,646, since the dot image printed in the forward path of the print head and the dot image printed in the return path are combined, the entire portion of the dot image of characters or figures which do not exceed the printing height of the print head cannot be printed only in one of the forward path and return path of the print head.